The present invention relates to instruments for use in the implanation of a knee prosthesis.
When fully extended, a vertical axis drawn through the centre of the femoral head passes through the centre of the knee joint and along the tibial medulla. This vertical axis is referred to as the mechanical axis, and it is along this axis that load is transmitted.
However, the femoral medulla lies at an angle of 6.degree. to 8.degree., typically 7.degree., to this mechanical axis.
It is important therefore when implanting a knee prosthesis to maintain a horizontally aligned bearing surface between the tibial and femoral components so that load is transmitted as in a natural joint.
Currently known methods of ensuring the correct positioning of the femoral and tibial components on the femur and tibia respectively involve the use of an external guide rod which is fixed to the outside of the leg prior to surgery, using the vertical axis passing through the centre of the hip as a visual guide. With such a method it is extremely difficult to cut bone sufficiently accurately from the natural joint so that the prosthetic component is a precise fit and, for instance, is flush with a natural bone at its edges.